The drilling of subterranean wells involves assembling tubular strings, such as casing strings and drill strings, each of which comprises a plurality of heavy, elongated tubular segments extending downwardly from a drilling rig into a wellbore. The tubular string consists of a number of threadedly engaged tubular segments.
Conventionally, workers use a labor-intensive method to couple tubular segments to form a tubular string. This method involves the use of workers, typically a “stabber” and a tong operator. The stabber manually aligns the lower end of a tubular segment with the upper end of the existing tubular string, and the tong operator engages the tongs to rotate the segment, threadedly connecting it to the tubular string. While such a method is effective, it is dangerous, cumbersome and inefficient. Additionally, the tongs require multiple workers for proper engagement of the tubular segment and to couple the tubular segment to the tubular string. Thus, such a method is labor-intensive and therefore costly. Furthermore, using tongs can require the use of scaffolding or other like structures, which endangers workers.
Others have proposed a running tool utilizing a conventional top drive assembly for assembling tubular strings. The running tool includes a manipulator, which engages a tubular segment and raises the tubular segment up into a power assist elevator, which relies on applied energy to hold the tubular segment. The elevator couples to the top drive, which rotates the elevator. Thus, the tubular segment contacts a tubular string and the top drive rotates the tubular segment and threadedly engages it with the tubular string.
While such a tool provides benefits over the more conventional systems used to assemble tubular strings, it also suffers from shortcomings. One such shortcoming is that the tubular segment might be scarred by the elevator gripping dies. Another shortcoming is that a conventional manipulator arm cannot remove single joint tubulars and lay them down on the pipe deck without worker involvement.
Other tools have been proposed to cure these shortcomings. However, such tools are often unable to handle tubulars that are dimensionally non-uniform. When the tubulars being handled are not dimensionally ideal, such as by having a varying wall thickness or imperfect cylindricity or circularity, the ability of tools to adequately engage the tubulars is decreased.